An optical modulator is one of optical waveguide devices using electro-optic crystal, such as a substrate made of LiNbO3 (LN) or LiTaO2. An optical modulator can be fabricated by, for example, forming an optical waveguide on part of the crystalline substrate and then disposing electrodes in the vicinity of the optical waveguide. Here, the optical waveguide is formed through thermal diffusion of a metal layer such as Ti formed on part of the substrate or through proton exchange in benzoic acid after the patterning.
One of such optical modulators includes a Mach-Zehnder optical interferometer which is formed on a crystalline substrate and that has a pair of interfering arm waveguides, and additionally includes a signal electrode and ground electrodes in the vicinity of the interfering arm waveguides to serve as coplanar electrodes.
For example, input light is introduced into the Mach-Zehnder optical interferometer and an electric signal (voltage signal) is applied to an electrode to interact with each other. As a result of the interaction, the refractive index of the Mach-Zehnder optical interferometer is varied, so that a modulated optical signal is output.
To cope with surge in transmission capacity, an optical communication system has issues of, for example, improvement in high-frequency characteristics of an optical modulator; low energy consumption due to enhance electric-field application efficiency; and cost reduction of device materials.
In an optical modulator, a portion at which a signal electrode is disposed along the interfering arm waveguides is a portion (interacting portion) at which the interaction occurs. For example, with the intentions of improving the efficiency of application of an electric field to the waveguide and also reducing material costs for electrodes, there are proposed techniques each in which the ground electrodes are formed so as not to cover the entire width of the substrate in the interacting portion of an optical modulator so that the interacting portion has a portion not covered with an electrode (e.g., Patent Literatures 1 and 2).
[Patent Literature 1] Japanese Laid-open Patent Publication No. 11-237593
[Patent Literature 2] Japanese Laid-open Patent Publication No. 2003-233048